"-dom" is a suffix (a group of letters at the end of a word to form a new word). A word ending in -dom means: an area, a domain, a group of attitudes, a group of people, etc. Examples include: kingdom, officialdom, boredom. English-speakers make up new -dom words all the time. Here, someone means the attitudes and behaviours of nerds. Since nerd ends with a d and dom starts with a d, one d is removed: nerd + dom --> nerdom.
The writer knows about nerdom and has now discovered a new, large area of nerdom.